Ethylene, propylene and BTX are important basic materials in producing petrochemical products. Ethylene and propylene are typically produced by steam thermal cracking of hydrocarbons mainly consisting of such paraffinic compounds as natural gas, naphtha and gas oil at a temperature of at least 800° C. in the absence of a catalyst. BTX is obtained as byproduct in this high-temperature thermal cracking. In order to improve yield of ethylene and propylene in steam thermal cracking of hydrocarbons, it is necessary to increase transition rate of hydrocarbon or selectivity of olefins. However, pure steam thermal cracking is limited in increasing transition rate of hydrocarbon or selectivity of olefins. Thus, a variety of methods have been proposed in order to increase production yield of olefins.
Methods of using catalysts in steam thermal cracking of hydrocarbons were proposed to improve yield of ethylene and propylene. U.S. Pat. No. 3,644,557 disclosed a catalyst comprising magnesium oxide and zirconium oxide. U.S. Pat. No. 3,969,542 disclosed a catalyst comprising calcium aluminate as basic component. U.S. Pat. No. 4,111,793 disclosed a manganese oxide catalyst supported on zirconium oxide. European Patent Publication No. 0212320 disclosed an iron catalyst supported on magnesium oxide. And, U.S. Pat. No. 5,600,051 disclosed a catalyst comprising barium oxide, alumina and silica. However, since these catalysts require high temperature for the steam thermal cracking of hydrocarbons, coking of the catalyst is severe.
U.S. Pat. No. 5,146,034 obtained olefins in high yield from low paraffinic hydrocarbons like propane using a ZSM-5 zeolite catalyst modified with a group 1A modifier. U.S. Pat. No. 5,968,342 disclosed a method of preparing ethylene and propylene in high yield using a ZSM-5 zeolite catalyst modified by an alkaline earth metal ion. There are many other patents using zeolite-based hydrocarbon cracking catalysts. Particularly, Japanese Patent Publication Nos. 11-253807 and 11-255674 disclosed a ZSM-5 catalyst modified by a rare earth metal element for increasing selectivity of olefins and a method for preparing olefins using n-butane. These patents are characteristic in that yield of low olefins is increased while minimizing production of aromatic hydrocarbon byproducts. However, they are disadvantageous in that yield of BTX is very low because the rare earth metal element used to modify the zeolite is present not only on the surface of the zeolite but also inside the pores, since a salt containing the rare earth metal element is used as precursor.
Until now, hydrocarbon cracking catalysts were mainly prepared by impregnation method or ion exchange method. When metal is added to the resultant zeolite catalysts, it tends to be present inside the pores of the zeolite, thereby greatly affecting inherent acidity of the zeolite.
In the conventional metal impregnation method, such salts as nitrate and sulfate are as used as metal precursor and water, alcohol, etc. are used as solvent (see FIG. 2). Thus, the metal component is present in the as ion precursor solution. The metal ion penetrates into the pores of ZSM-5 and exists not only on the surface of ZSM-5 but also inside the pores, after calcining. The resultant catalyst has significantly reduced acidity and thus has poor hydrocarbon cracking activity.
In hydrocarbon cracking using MFI type zeolites like ZSM-5, yield of BTX tends to be low when yield of olefins such as ethylene and propylene is high, and vice versa. This is because the product distribution is closely related with strength and amount of acid sites of the ZSM-5 catalyst. In order to maximize product value in hydrocarbon cracking, a catalyst capable of increasing yield of BTX as well as that of olefins is required.